The KCNE family of accessory channel subunits associate with a variety of voltage gated channels to regulate their assembly and function. Mutations in KCNEs have been linked to hereditary arrhythmias of the Long QT syndrome (LQTS), atrial fibrillation, and polymorphisms may contribute to drug-induced ventricular arrhythmias. Moreover, the KCNEs are increasingly found to regulate the activity of numerous channels in systems beyond the heart. Progress in the study of these proteins has revealed much regarding their function, expression and genetics. Nevertheless many unresolved issues remain. Among these are the precise mechanisms of regulation of channel gating, their assembly with K channels, molecular basis for pro-arrhythmic mutations, relative preference for specific channels and stoichiometry. Our previous work has shown that portions of the KCNEs separate from those that govern activation may influence regulation of channel deactivation. Our preliminary studies indicate that C-termini of KCNEs and KCNQ1 channels physically and functionally interact to alter channel deactivation rates and voltage-dependence of activation. Here we propose to address several of the unresolved issues concerning control of channel deactivation rates and several newer questions pertaining to the broader KCNE family that may influence cardiac arrhythmia risks. Our approach will use biochemical, functional and structural methodology. We propose more advanced structural analyses on KCNE regulation of KCNQ1 channels and how LQT mutations alter this process.